1. Field of the Invention
This invention relates to switching power supplies which employ pulse-width modulation, and in particular to a current limiter therefor.
2. Background Description
With the advent of the transistor and subsequently the introduction of integrated circuits, the demand for small, light-weight, regulated low-voltage power supplies required that substantial changes were necessary in power supply design. Today, the majority of power supply specifications require that relatively low voltage DC outputs, usually stabilized, be derived from 50 or 60 Hz AC mains. Conventional power supplies do not meet present day requirements because such power supplies usually include a heavy, bulky and expensive mains frequency transformer, rectifiers, capacitors having a large storage capacity, and smoothing inductors which provide, in combination, energy storage and smoothing and linear series regulators, which are inefficient and bulky. In addition, very large heat sinks are required in order to dissipate energy so as to cool the power dissipating components. In such conventional linear regulators, something like 80% of the bulk of the unit is accounted for by three things. The mains transformer, the electrolytic capacitors, and the heat sinks required to cool the power-dissipating components consisting of rectifiers and series-path transistors.
The problem of size and weight can be reduced if it is possible to increase the operating frequency from that normally found in conventional AC mains. For this reason, the pulse-width-modulated power supply was developed. In such a power supply, a DC voltage is obtained from a DC power source. This DC voltage may be derived by line rectification and transformation from the AC mains input. The output of the DC power source is applied directly to a switching inverter. Typically, such inverters include switching transistors connected either in a bridge, half-bridge or push-pull arrangement, whereby a square-wave pulse output is applied to the primary of a transformer. The on-time of the switching transistors is controlled by a pulse-width modulator which combines the functions of regulation with control of the inverter. A DC feedback circuit from the output of the power supply, which provides a measure of the output voltage, is applied as one input to a comparator, which is compared against a reference voltage so as to cause the pulse-width modulator to change the on-time of the switching transistors in order to maintain the regulator output voltage. The pulse-width modulator operates under control of a clock which may operate at a frequency in the order of 40 kHz. Such pulse-width-modulated power supplies are well-known, and one such is described in an article entitled, "Switching Power Supplies: Why and How", by Malcolm, Burchall, p. 73-75, Electronic Engineering, September 1973. A pulse-modulated power supply operating on a clock frequency of 40 kHz is described in an article entitled "Use a Pulse-Width-Modulated Switcher", J. H. O'Neal, vol. 25, no. 8, Electronic Design, Apr. 12, 1977, p. 110-115, and present-day power supply designs are described in the text entitled "Switching and Linear Power Supply, Power Converter Design", Abraham I. Pressman, Hayden Book Company Inc., 1st printing 1977.
In addition to controlling the on-time of the switching transistors in the inverter circuit, it is necessary to have a current-limiter mechanism in order to avoid destruction of components, such as the switching transistors. In the prior-art systems, it is not uncommon to sense the current in the inverter circuit rather than at the output of the power supply, because the current amplitude is normally less, and there is effective protection where it is most useful. However, the prior-art circuits normally use either a full bridge or one-half bridge configuration, and in these circuits, the current is sensed using a resistively loaded current transformer in series with the primary of the output transformer of the inverter. A number of disadvantages accrue by the use of the current transformer, which is necessary for isolation purposes. First, the transformer removes DC information from the current pulses, which necessitates some form of DC restoration or rectification after the transformation. Further, the time constants in the current-sensing loop are longer than the current pulse width and thus allows for integration of the switching element current pulses. This creates the possibility of an over-current condition occurring in the switching elements before the current-limiting circuit has time to respond to a current overload condition. Further, where latching-type circuits are employed to turn off the power supply on the occurrence of the first overcurrent condition, and subsequently allowing the power supply to attempt to restart, problems are introduced when the power supply cycle is turned on and off.